14 - What are Bacteria? (w/ Neeraj Karamchandani!)
14. What are Bacteria? (w/ Neeraj Karamchandani!)
Imagine having a twin split out of you in twenty minutes flat; each of you split again in another twenty minutes. In a scant few hours, there are billions of you. This happens with bacteria every day in almost every environment one can imagine. Just what is this domain of life? Why are bacteria important to our global ecosystem? Are they always dangerous? Let’s learn to be scientifically conversational.
General Learning Concepts
1) What is a bacterium? How are they different than humans?
a. What is a bacterium? Microscopic, single-celled organisms. They do not have different internal compartments like Eukaryotic cells (human cells, plant cells, yeast cells, for example). They have a cell wall of some thickness, unlike human cells. They can be found in all manners of locations (mountains, oceans, inside of animals, frozen in ice, just about everywhere). There are likely 1 trillion different types of microbes on Earth and 99.999 percent have yet to be discovered.
b. What do bacteria need to live? Some require oxygen to make energy, some don’t. Some require organic carbons (like decaying material or being able to do fermentation) and some don’t (can use carbon dioxide as a source of inorganic carbon which can be fueled by light energy). Bacteria acting as decomposers are quite critical to our environment as we know it.
c. Why are bacteria important to our global ecosystem? It is unlikely that life as we know it would exist without bacteria. Organic carbon would remove all of the carbon dioxide in the atmosphere if bacteria didn’t break them down and decompose them. If there was no carbon dioxide, plants could not photosynthesize. This immediately would cause a trophic breakdown where primary producers would be gone and trophic predators (including humans) would immediately starve to death and die out. Decomposition on its own releases nutrients into the environment that can be used for a myriad of purposes. Bacteria are also quite important for nitrogen fixation (making nitrogen that is usable for plants).
d. Are bacteria yeast? Or fungi? No, neither! Fungi are Eukaryotes, not bacteria. Just because something is single-celled doesn’t mean it’s a bacterium. Amoebas, yeast, and members of Archae are not bacteria.
2) Much of what we knew early from bacteria was because of disease
a. Germ Theory and Robert Koch: Once upon a time, illness as believed to have come from “evil spirits” or foul orders. It was once incredibly rare to have children live to adulthood. Louis Pasteur (1822 – 1895) was an unparalleled scientist in many ways, studying bacteria, diseases of beer and wine, anthrax, cholera, silk blight, etc. Robert Koch (1843 – 1910, 1905 Nobel Prize for Physiology or Medicine) was famous for many other discoveries but is now well known for developing Koch’s postulates. Both men independent worked to show that Bacillus anthracis was responsible for producing anthrax; Koch’s postulates came from showing that cultures of bacteria from blood of infected animals could be communicable (using mice as model organisms).
i. Koch’s Postulates: 1) the microorganism or other pathogen must be present in all cases of the disease 2) the pathogen can be isolated from the diseased host and grown in pure culture 3) the pathogen from the pure culture must cause the disease when inoculated into a healthy, susceptible laboratory animal 4) the pathogen must be reisolated from the new host and shown to be the same as the originally inoculated pathogen
ii. These postulates are flawed: The cholera causing agent Vibrio cholerae could be isolated from both sick and health people (against postulate 2). Many viruses do not cause illness in all individuals (postulate 1) and infections with the same virus can cause different illness (PV infection only causes paralytic disease in 1% of the infected). It is challenging to grow all disease-causing microbes in pure culture (2 and 3). There are now more up-to-date postulates for the 21st century by David Fredricks and David Relman from Stanford University School of Medicine.
b. Bacteria have a fossil record: Yes, they’re small, but they can leave fossils. Specifically, cyanobacteria have left fossil records back to the Precambrian (3.5 billion years old) which dates among some of the oldest known fossils. They also have a property that allows them to form large, layered structures (which reminds us of biofilms, which we will discuss next week!). Cyanobacteria are aquatic and are capable of photosynthesizing. They were incredibly important for the oxygen atmosphere that we so appreciate today. Large mats of the bacteria are called “blue-green algae”, but once again do not confuse bacteria with algae. They are not eukaryotes. Additionally, the chloroplast that plants use to photosynthesize is essentially a cyanobacterium that lives within the plants’ cells. We’ll discuss this more in the episode about the endosymbiotic theory.
3) Why are bacteria useful in molecular biology? Why are they potentially dangerous?
a. Useful, molecular cloning: We have already discussed in episode two how bacteria are capable of being used for molecular cloning. Still, it bears repeating that E. coli is capable of dividing into two separate cells in 20 minutes, even though it takes 40 – 90 minutes to replicate its genome. What is seen is that when each daughter chromosome segregates into the daughter cells, they’re already partially replicated for their next daughter cells. The rapidity of E. coli replication is one of the reasons that they’ve become so useful for molecular biology, like insulin production.
b. Useful, CRISPR: Clustered Regularly Interspaced Short Palindromic Repeats, or CRISPR, are a bacterial defense system (though originally discovered in archaea). Viruses that infect bacteria, called bacteriophage, can be prevented by leaving remnants of their genetic code in the host bacterium. Essentially, that allows for the system to be a genetic memory that helps the cell detect and destroy those viruses. Since then, humans have used elements of CRIPSR to edit genomes of mouse and human cells with precision and ease.
c. Dangerous: Many bacteria cause disease or illness: salmonella from Salmonella typhimurium (recently in the news due to a rash of outbreaks from pet hedgehogs), Vibrio cholerae (cholera, a diarrheal illness caused by some strains like O1 or O139), Legionnaires disease from Legionella pneumophila (famous for a rash of deaths in Philadelphia in 1976 from inhaling bacteria on small droplets of water in the air), or typhoid fever (Salmonella typhi). It is important to remember that while there are many bacteria that elicit fear because of symptoms, a majority of bacteria have no ill effects and many live on us and in us.
4) Fun Tidbits
a. Quorum sensing: Bacteria may be single-celled organisms but they are capable of speaking to one another. Essentially, they do this in a way that is somewhat similar to how our cells (human cells) speak to one another, like how we discussed insulin being a hormone in episode 2. They use signaling molecules and are capable of measuring the number of molecules in the area, which allows for greater flexibility of communication. We will discuss more about quorum sensing next week when we discuss biofilms.
b. The human microbiome: Briefly discussed in episode four, the microbiome is a community of important groups that play roles in keeping our bodies healthy. These microorganisms include not only bacteria but fungi, parasites, and viruses. This is another great example that not all microorganisms are dangerous and we rely on many (dangerous and not) to coexist in our bodies. This microbiome stimulates the immune system, helps us digest food, synthesizes vitamins and amino acids, break down starches and fibers. We will discuss the microbiome in more detail when we discuss Crohn’s disease and related illnesses.
5) Solicited Naïve Questions
a. Is all E. coli dangerous? Just as all of the other topics we’ve touched on today, the answer is no. Most E. coli are harmless and are important for the human intestinal tract and to be a part of the microbiome. However, if the bacteria got out of the intestinal tract it could cause issues in a different location. One of the more famous examples of dangerous E. coli is Shiga toxin-producing E. coli, which can cause stomach cramps, diarrhea, and vomiting.
b. How do bacteria move? Some have thin, whiplike structures called flagella that rotate like a boat engine to push the cell forward. Some have pili, small hook like appendages that bacteria use to pull themselves along. Some join biofilms, which we will discuss in episode 15.